In recent years, fuel cells using a solid polymer membrane as an electrolyte have attracted attention because they can be made smaller and lightweight and provide a high output density even at comparatively low temperatures, prompting acceleration of their development.
Solid polymer materials used for this purpose are required to have superior proton conductivity, suitable moisture retention and impermeability to gases such as hydrogen and oxygen. Various studies have been conducted on polymers having groups such as sulfonic acid groups and phosphonic acid groups as materials that satisfy these requirements, and numerous such materials have been proposed (refer to, for example, O. Savadogo, Journal of New Materials for Electrochemical Systems I, 47-66 (1998)).
However, under actual fuel cell operating conditions, active oxygen species are generated at the electrodes and have a high level of oxidizing power. Thus, in order for the fuel cell to operate in a stable manner, and over a long period of time in particular, it is required to be durable in this type of harsh oxidizing atmosphere. Although numerous hydrocarbon-based materials have been reported that demonstrate superior characteristics with respect to characteristics of the fuel cell during initial operation, thus far they have problems in terms of oxidation resistance.
Consequently, perfluorosulfonic acid polymers having the repeating unit shown in formula (15) below are currently mainly being used in studies targeted at practical application.
(wherein, k and l represent integers such that k/l is 3 to 10, p is 0 or 1, and q is 2 or 3).
In addition, in fuel cells, not only the membrane, but also the catalyst surface of the gas diffusion electrode, are required to have continuous proton conductivity, and similar perfluorosulfonic acid polymers are used as the binder of this electrode catalyst as well.
On the other hand, as bis-sulfonylimide groups are known to demonstrate greater superacidity as compared with sulfonic acid groups, polymers having bis-sulfonylimide groups instead of sulfonic acid groups have expected to become new materials for electrochemical processes, although they are still under development.
These fluorinated sulfonic acid polymers and fluorinated bis-sulfonylimide polymers are referred to as superacid polymers because their acidic groups demonstrate stronger acidity that even sulfuric acid.
First, an explanation is provided of the background art relating to perfluorosulfonic acid polymers.
The aforementioned perfluorosulfonic acid polymers are obtained by carrying out a hydrolysis reaction on a copolymer of a perfluorovinyl ether represented by the following formula (16) and tetrafluoroethylene (TFE).
(wherein, p and q are defined in the same manner as in the aforementioned formula (15)).
A technique is normally used for the hydrolysis reaction in which, after having converted a polymer, in which the end of the side chain is in an —SO2F form, to a sulfonic acid salt form using a base such as NaOH or KOH, it is further converted to an —SO3H form by an acid such as hydrochloric acid.
In the case of using this polymer as a membrane, methods are known in which the hydrolysis reaction is carried out after forming a membrane out of a polymer, in which the end of the side chain is in the —SO2F form, by heat molding (a melt molding membrane formation process), or a solution or dispersion, in which an —SO3H type polymer has been subjected to dissolution treatment, is formed into a membrane by casting (a casting membrane formation process). In addition, in the case of using it as a catalyst binder, a method in which a solution or dispersion of —SO3H type polymer is coated onto a catalyst layer, or a method in which said solution or dispersion is mixed with a catalyst and then coated onto a substrate followed by drying, is typically employed.
Among these methods, as the melt molding membrane formation process causes extensive swelling of the membrane during the course of hydrolysis, it presents difficulties in handling. On the other hand, although the casting membrane formation process, or a solution or dispersion for use in a binder, involves dissolution treatment of a sulfonic acid polymer, it is necessary to hydrolyze an —SO2F type polymer in advance. At that time, it was necessary to first convert to a sulfonic acid salt form with base followed by conversion to a sulfonic acid form by acid treatment and finally washing with water as previously described. As it is necessary, in particular, to carry out acid treatment completely to prevent any salt from remaining, the process involving this alkaline hydrolysis had numerous complex steps.
Furthermore, as the spacer portions between the main chain and sulfonic acid groups are shorter in the case p=0 than in the case p=1 in the polymer represented by the aforementioned formula (15), it demonstrates a high glass transition temperature and high strength, and is also preferable in terms of heat resistance. However, the yield of the monomer represented by the aforementioned formula (16) (p=0), that serves as its raw material, is extremely low due to an extensive side reaction in the form of a cyclization reaction that occurs in its production process. In the case q=3, for example, although a fluorinated monomer is obtained, its yield is at most about 50% due to the occurrence of a side reaction in the form of a cyclization reaction, and in the case q=2, only the cyclization reaction proceeds, preventing any fluorinated monomer being obtained.
Therefore, in order to solve the problems associated with monomer synthesis in the case p=0 in formula (16), a fluorinated monomer having a sulfonic acid precursor group capable of being derived to sulfonic acid has been proposed for the structure other than the —SO2F groups. For example, although International Unexamined Patent Publication No. 98/43952 and Japanese Examined Patent Publication No. 47-2083 describe a production process of a monomer in which the functional groups are replaced with sulfonic acid salts, this salt-type monomer cannot be purified by distillation since it lacks volatility, thereby making it difficult to obtain a highly pure product, and also has the problem of difficulty in membrane formation following polymerization. In addition, although a method has also been proposed in which a salt-type monomer is converted to an —SO2F form, this method was excessively complex.
In addition, although a methyl ester form is described in Japanese Unexamined Patent Publication No. 61-133211 as an example of monomers and polymers having a sulfonic acid ester structure, as methyl esters of sulfonic acid degrade in the presence of humidity and have such high levels of reactivity that they are used as alkylating agents, they have not been used practically due to handling difficulties in air.
On the other hand, monomers and polymers containing bis-sulfonylimide groups were first synthesized by DesMarteau, et al. For example, the following monomers and polymers are reported in U.S. Pat. No. 5,463,005.

With respect to the monomers of (17), (18) and (19), copolymers with TFE are synthesized by aqueous emulsion polymerization, and the results of evaluation for use as a fuel cell membrane for a copolymer with (18) are reported in the literature (DesMarteau, et al., Polym. Mater. Sci. Eng. 1999, 80, 600) as having an ion exchange capacity represented with equivalent weight (to be represented by EW, with smaller values indicating larger ion exchange capacities) of 1175 to 1261 g/eq. In addition, ionomers having several types of bis-sulfonylimide groups, including a copolymer with (18) that is described in the literature (Zhou, Ph.D. thesis 2002, Clemson Univ.) as having an EW of 1175 g/eq, are reported to demonstrate higher thermal stability than ionomers having sulfonic acid groups. Moreover, a copolymer of the monomer of formula (21) below and TFE described in the literature (Xue, Ph.D. thesis 1996, Clemson Univ.) as having an EW of 970 g/eq is synthesized by aqueous emulsion polymerization.CF2═CFOCF2CF2SO2NHSO2CF2CF2CF2CF3  (21)
In all of these reports in the literature, copolymerization with TFE is carried out by aqueous emulsion polymerization after having converted a bis-sulfonylimide group-containing monomer into a salt form, and typically only polymers having a large EW (of nearly 1100 g/eq or more) have been obtained. Alternatively, polymers having a small EW were first obtained by increasing the size of the terminal perfluoroalkyl group in the manner of copolymerization with monomer (21). (However, an excessively large terminal perfluoroalkyl group results in the problem of lowering Tg, and since it is also difficult to form a cluster of bis-sulfonylimide groups, there is the disadvantage of low proton conductivity relative to EW.) This is thought to be due to it having been difficult to create polymers having a high ion exchange group density (low EW) due to the low solubility in other fluorine-containing monomers in the case of monomers having a short terminal perfluoroalkyl group or short chain monomers having short spacer sections and, particularly, in the case of having been converted to a salt form.
In addition, if bis-sulfonylimide groups are left in acidic form, as it becomes difficult for the polymerization initiator to dissolve in a fluorine-containing solvent, or degradation may result due to the action of strong acid in the case of polymerization initiators like fluorine-containing diacylperoxides, it was not possible to carry out solution polymerization at a high monomer concentration using fluorine-containing solvents.
On the other hand, a process is described in Japanese Unexamined Patent Publication No. 2002-212234 in which a bis-sulfonylimide group-containing polymer is produced by reacting a polymer having an —SO2F terminal of the prior art with, for example, CF3SO2NH2. In this process, although it is possible to produce a polymer having high density of ion exchange groups by using a polymer having high density of —SO2F groups, in this case, as a side reaction in the form of the reaction with water is much faster, it is difficult to avoid the presence of sulfonic acid groups that lower thermal stability in the resulting polymer, thereby impairing the thermal stability that characterizes bis-sulfonylimide group-containing polymers. In addition, although the resulting bis-sulfonylimide group-containing polymer is obtained in the form of a tertiary amine salt or another salt, as the salt is difficult to remove, it is difficult to completely convert to the acid form.
Thus, a process has heretofore not been known for efficiently producing a highly pure bis-sulfonylimide group-containing polymer having a high density of ion exchange groups and high thermal stability as a result of not containing sulfonic acid groups.
Furthermore, International Unexamined Patent Publication No. 03/050151 describes the general formula of an ionomer containing a bis-sulfonylimide group, and although the structure of an ionomer containing a partially fluorinated alkyl group having 2 to 10 carbon atoms as the N terminal group is disclosed therein, no further detailed description is provided. Namely, there are no descriptions contained in International Unexamined Patent Publication No. 03/050151 relating to a monomer, its production process or its utilization methods of the present application.
A method for forming a membrane by casting, in which a dimethylformamide (DMF) solution of a bis-sulfonylimide group-containing ionomer is employed, is described in the literature (DesMarteau, et al., Langmuir 2000, 16, 8031). However, in the case of a DMF solution, it is difficult to completely remove the solvent due to its high boiling point. Moreover, DMF is partially degraded by a strong acid such as bis-sulfonylimide and, as the degradation product is toxic to fuel cell catalysts, it could not be used in applications such as the production of gas diffusion electrodes. Thus, a solution of a bis-sulfonylimide group-containing polymer that is suitable for production of casting membranes and production of gas diffusion electrodes has heretofore not been known.